In order to realize harmony between human activities and the environment, it is necessary to reduce contaminants generated by human activities as much as possible and to treat generated contaminants to make them harmless. Therefore, industrial water or the like is conventionally treated by oxidizing ammonium nitrogen contained therein to nitrite nitrogen/nitrate nitrogen by an aerobic biological nitrification process and then reducing it to nitrogen gas by an anaerobic denitrification process. Nitrification (oxidation reaction) and denitrification (reduction reaction) are opposite reactions, and therefore it is usually difficult to perform both these reaction in one reactor.
For this reason, a reactor is generally separated into two tanks, a nitrification tank and a denitrification tank, to sequentially treat water in these tanks (Non-Patent Literature 1). For example, when the nitrification tank (aerobic treatment tank) and the denitrification tank (anaerobic treatment tank) are arranged in this order, the reactor has a high ability to remove nitrogen from water so that the quality of final treated water is improved. However, it is necessary to add a carbon source (hydrogen donor), such as methanol, required for denitrification. The amount of the carbon source (hydrogen donor) required for denitrification is determined on the basis of the amount of nitrogen contained in water, and therefore the supply of the carbon source (hydrogen donor) needs to be strictly controlled. From an industrial viewpoint, an excessive amount of the carbon source (hydrogen donor) is added to perform denitrification. Therefore, equipment for finally treating the excess carbon source (hydrogen donor) is often provided.
As a method that makes it possible to omit equipment for controlling the supply of a carbon source (hydrogen donor) or equipment for post-treating a carbon source (hydrogen donor), a method is known in which a denitrification tank (anaerobic treatment tank) and a nitrification tank (aerobic treatment tank) are arranged in this order to cyclically return a nitrified liquor obtained in the latter part to the denitrification tank (anaerobic treatment tank) of the former part to make it easy to control the supply of a carbon source (hydrogen donor) (Non-Patent Literature 1), or a method is known in which a biodegradable plastic is provided as a carbon source (hydrogen donor) in an anaerobic treatment tank and/or an aerobic treatment tank (Patent Literature 1). However, these methods have problems that circulation equipment is required, the sizes of tanks are increased to provide residence time, and the quality of final treated water is lower than that of water treated first in a nitrification tank and then in a denitrification tank.
Further, nitrite nitrogen (N—NO2) generated by nitrification inhibits nitrification. Therefore, when nitrite nitrogen is accumulated in a system, a nitrification rate is reduced so that nitrification does not sufficiently proceed (see Non-Patent Literature 2).
In order to avoid this, a method has been proposed in which water to be treated is previously diluted before water treatment to reduce the concentration of ammonium nitrogen in the water to be treated. However, this method has a problem that when wastewater having a high ammonium nitrogen concentration is treated, the volume of water to be treated is increased so that the sizes of tanks are increased and therefore the cost of equipment is increased.
On the other hand, an ammonia stripping method is known as a physicochemical treatment method. This method can be performed more stably than a biological treatment method, but has problems that the cost of energy is high due to the use of steam and addition of an alkaline agent is absolutely necessary to make water to be treated alkaline.